In an art object and a craft object, color is one of important characteristics. However, a painting and a pot themselves do not emit light. Accordingly, when a showpiece is appreciated in an art gallery, a museum, or the like, illumination is as important as a work. That is because an appreciator of the showpiece observes light reflected on a surface of the showpiece in visible light irradiated from an illumination light source. Accordingly, no matter how beautiful color an artist expresses, as long as light irradiated to the showpiece does not contain a light-emitting component corresponding to a specific color, the showpiece is observed as a color tone which is dim and poor in a color sense by the appreciator, for example.
A most preferable light as light which is used for illuminating such a showpiece in the art gallery, the museum, or the like is sunlight. The sunlight is constituted of wavelength components of continuous light and almost equally contains all light components from 400 nm to 760 nm which are visible light wavelengths. Accordingly, the sunlight makes it possible to reproduce colors of all substances which exist in nature as original colors in the substances. However, even when the sunlight is excellent as light for an application to the above-described illumination, a high-class art object such as the painting is not often appreciated under direct exposure to the sunlight in bright space outdoors, for example. One of reasons why the art object is housed and appreciated in a specific location such as the art gallery is because it is kept from wind and rain and an accident such as a theft.
As an important reason other than the above-described reason, protection of the art object from a large amount of light irradiation can be cited. This is because the sunlight contains the visible light of all wavelengths and contains the light-emitting components other than the visible light, such as ultraviolet light and infrared light. Because in particular, energy of the ultraviolet light is higher than energy of the visible light, the direct exposure of the art object such as a historic painting to the sunlight accelerates, for example, color fading, brittleness, or the like, for example. Therefore, an artificial light source is required, and in addition to convenient features of artificial light, in which light intensity can be adjusted, ultraviolet amount is reduced as much as possible, and the like, a light source capable of reproducing sunlight faithfully is required, for example.
In recent years, from the viewpoint of energy saving and reduction in an amount of carbon dioxide emission, a light source using a light emitting diode (LED) has received attention as the artificial light source. The LED has a longer operating life and saves energy compared with a conventional incandescent light bulb using a tungsten filament. Further, a market for LED lighting is growing rapidly owing to convenience of the LED. As conventional LED lighting, there is a lot of LED lighting of a type in which white light is obtained by combining a blue LED and a yellow phosphor. The above-type LED lighting has been short of warmth of light, and light of the LED lighting has been unnatural white light. LED products remarkably improve in performance with expansion of the market. Accordingly, thereafter, as a result of improvement regarding the combination of the LED and the phosphor, a white light source capable of reproducing light equal to sunlight is being developed.
As the white light source which emits light of an emission spectrum equal to the sunlight, the white light source in which the sunlight with different color temperatures is reproduced by a black body radiation spectrum of the same color temperature can be cited, for example. In the above-described white light source, not only apparent white light but also white light approximating the sunlight in an emission spectrum shape of the white light can be obtained with respect to the sunlight whose color temperature changes with time. Further, as a lighting system using the white light source, the lighting system such as office lighting in which the center of an object of lighting is a human or the like can be cited, for example. The above-described lighting system is a system which allows adjustment of a color temperature and illuminance of indoor light while detecting a change in outdoor light. The above-described lighting system makes it possible to obtain white light corresponding to a human physiological phenomenon and a change in seasons, for example. Further, as a system using the white light source, an artificial sunlight system in which a plurality of light emitting diode modules with different color temperatures are combined with one another can be cited. The above-described artificial sunlight system allows reproduction of a change in the color temperature of sunlight irradiated at points whose latitude and longitude are different on the earth.